fix(upload): preserve fields in multipart fallback

- Recover regular multipart fields when the file part is not exposed through request.files.

- Preserve expires and note values for browser uploads that need body-based file recovery.

- Add parser coverage for file parts without filenames and recovered expiry fields.

- Extend upload route tests to assert recovered uploads keep expiry metadata.

- Bump NanoShare to 1.27.3.

tests/test_upload_content.py

@@ -0,0 +1,28 @@
+from my_modules.upload_content import parse_multipart_upload_body, recover_file_from_multipart_body
+
+def test_recovers_file_part_without_filename():
+  boundary = '----nanoshare-test-boundary'
+  body = (
+    f'--{boundary}\r\n'
+    'Content-Disposition: form-data; name="file"\r\n'
+    'Content-Type: application/octet-stream\r\n'
+    '\r\n'
+  ).encode() + b'hello upload' + (
+    '\r\n'
+    f'--{boundary}\r\n'
+    'Content-Disposition: form-data; name="expires"\r\n'
+    '\r\n'
+    '7d\r\n'
+    f'--{boundary}--\r\n'
+  ).encode()
+
+  uploaded = recover_file_from_multipart_body(body, boundary)
+  parts = parse_multipart_upload_body(body, boundary)
+
+  assert uploaded is not None
+  assert uploaded.filename == 'file'
+  assert uploaded.mimetype == 'application/octet-stream'
+  assert uploaded.stream.read() == b'hello upload'
+  assert parts.fields == {'expires': '7d'}
+  assert parts.file is not None
+  assert parts.file.stream.read() == b'hello upload'